Lithium Polymer batteries are the newest and currently one of the best choices for batteries in a portable. They are smaller than other batteries, don't develop a "memory" , and last through more charges/discharges than other types of batteries.

But working with Lithium Polymer batteries can be dangerous and precautions must be observed. Be very careful with polarity when hooking up Li-Po's. Also ALWAYS use a protection circuit (PCB) and a Lithium charger that is appropriate for the number of Li-Po's you will be using.

Lithium Polymer batteries come as single 3.7v cells. These cells can be packed together to get 7.4v , 11.1v, 14.8v , etc Li-Po Packs.
When using a 7.4v pack you will want a 7.4v Pcb and a 7.4v charger. Ditto on the rest.

The following diagrams will be based on wiring the 3.7v cells in series to add their voltages together to get the required voltages we need. Batteries wired in series are wired like so :

Ah, ok! From his picture for the 7.4v protection circuit, hook up both positives to the B+ part and both negatives to the B- part. The Com (common) won't matter, because that is only for batteries in series.